1. Field of the Invention
The present invention relates generally to a method and an apparatus for estimating and removing an echo channel. The present invention relates particularly to a method and an apparatus for estimating the echo signal formed when an output signal of a transmission antenna of a wireless repeater returns and is re-input via a reception antenna, and transmitting, via the transmission antenna, a signal from which the estimated echo signal has been removed in advance.
2. Description of the Related Art
Mobile communication plays an important role in the development of an information-oriented society. It is optimal for a mobile communication system to allow a subscriber to utilize the functions of an entire communication network wirelessly, so that the subscriber may communicate with anybody, anywhere, at anytime. The mobile communication system utilizes a wireless repeater for swift communication in a communication scheme based on wireless and mobile communication technologies, such as indoor/outdoor wireless data communication, vehicle and cellular phone, a future personal communication network, etc. The wireless repeater is equipment for primary use in a shadow region, such as a shielded space, an underground shopping center, a tunnel, and an underground parking lot, where it is not suitable to newly establish a base station or operate a relay vehicle. The wireless repeater extracts a weak signal to be relayed from signals still existing in a shielded space or a shadow region, amplifies the extracted signal with a low noise, and re-transmits the amplified signal via an antenna, thereby allowing mobile communication even in a communication obstacle region.
However, when the wireless repeater transmits a radio signal to a mobile terminal, oscillation or a fine oscillation is generated due to the fact that an output signal of a transmission antenna returns, is re-input via a reception antenna and is amplified, thereby deteriorating signal quality. Such a limitation can be resolved using a method of separating the transmission/reception antennas of the wireless repeater by a sufficient distance. However, this method causes the installation costs of a wireless repeater to increase and it is difficult to select a region where installation is possible in a scattering environment such as the downtown.
To prevent the above-described oscillation phenomenon without providing additional separation distance between a transmission antenna and a reception antenna, a technique is performed in which a signal returning from the transmission antenna to the reception antenna is estimated using digital signal processing in a baseband or an intermediate frequency band, and the estimated returning signal is removed. Due to time delay and costs, this technique generally uses a unique characteristic of a signal in order to estimate a returning signal without decoding a transmission/reception signal.
Since a spread spectrum scheme, such as Code Division Multiple Access (CDMA), uses a scrambling code, such as a pseudo noise code, the spread spectrum scheme has an autocorrelation characteristic where a discrete signal having a sampling frequency of a wireless repeater is excellent. Therefore, it is possible to prevent oscillation by an echo signal by estimating the echo signal using this autocorrelation characteristic and removing the estimated echo signal. However, in a wireless repeater where a multi-carrier system is applied that does not guarantee the autocorrelation characteristic, it is difficult to accurately estimate an echo signal and prevent oscillation.
FIG. 1 is a block diagram illustrating the construction of an apparatus for removing an echo signal and preventing oscillation in a general wireless repeater. Referring to FIG. 1, a multi-carrier input signal received via a reception antenna of a wireless repeater passes through a delay generator 101. The delay generator 101 compensates for a synchronization difference by an additional delay, such as a band pass filter required during analog/digital conversion, and is output to a transmission antenna. An echo signal is removed by applying an output signal of the delay generator 101 as an input of an echo signal remover 103, and subtracting a filter response of an echo signal estimator 105 from a multi-carrier input signal. For correct operation of the echo signal remover, an impulse response of an echo channel is estimated and used as a filter coefficient of the echo signal remover. When the multi-carrier input signal is in a time-division train signal section, a multi-carrier input signal is transferred to an input channel estimating unit through a switch. The input channel estimating unit includes an input channel estimator 107 and a train signal generator 109. The input channel estimating unit removes an input signal from an input signal of the echo channel estimator 105 by passing a train signal generated by the train signal generator 109 through the input channel estimator 107. When the above-described process is performed, an echo signal and an input signal are removed, so that only an additional noise is left. Therefore, when an estimation technique such as Least Square Estimation (LSE) and Minimum Mean Square Error Estimation (MMSE) is applied, an echo channel and an input channel filter coefficient can be estimated that minimize power sum of interference-removed output signal or average power.
However, since a train signal section of the multi-carrier system is an extremely small portion of an entire signal section, when a time-variant characteristic of an echo channel cannot be ignored due to an influence of neighboring scattering objects, in an aspect of an installation environment of a wireless repeater, quality of interference-removed signal may be deteriorated by a channel change between a periodic time-division train signal section and the other sections.